FIELD OF THE INVENTION
The invention relates to methods and apparatus for estimating rates and volumes of irrigation fluid absorption and/or tissue loss by a patient during a surgical procedure.
Physiological Fluid Compartments
Total body water, which represents about 45% of total body weight in women and about 50% of total body weight in men, may be regarded as occupying primarily three physiological volumes or compartments. The largest of these three compartments, intracelular water, accounts for about 30-35% of total body weight. The two smaller compartments, which together comprise extracellular volume, are interstitial volume (about 10-12% of total body weight) and plasma volume (about 4-5% of total body weight). Plasma volume, in turn, is about 60-70% of blood volume with the remainder of blood volume being substantially occupied by formed elements such as red and white blood cells.
During surgery, optimal patient care requires maintaining the plasma/blood ratio, plasma osmolality, and plasma electrolyte levels within fairly narrow ranges. Note that if a portion of the plasma is replaced by an equal volume of water, the plasma/blood ratio may stay constant while plasma osmolality and electrolyte concentrations will decrease. On the other hand, adding water to a fixed volume of blood will both dilute the plasma (thus lowering its osmolality and electrolyte levels) and increase the plasma/blood ratio. Keeping plasma osmolality in a normal range helps assure that cells neither lose nor gain too much water by osmosis, while maintaining plasma electrolyte levels is important for normal cellular function (especially in neurons). The plasma/blood ratio strongly affects both blood flow characteristics and oxygen delivery to the tissues. Thus, significant blood loss and/or fluid absorption in surgical patients is preferably monitored in real time to detect unfavorable changes and allow timely corrective action.
Blood Loss and Fluid Absorption During Surgery
Blood lost during open surgery usually is blotted with surgical sponges or washed away from the surgical site with a stream of sterile irrigation fluid. Clinically significant blood loss may be monitored by weighing sponges and visually estimating the amount of blood flowing into the wound. During endoscopic surgery, on the other hand, virtually all lost blood is carried away in the irrigation fluid where it is difficult to quantify.
Nevertheless, endoscopic and other substantially closed procedures (including liposuction) which carry a risk of significant blood loss are becoming more common. Previously well-known endoscopic procedures (such as hysteroscopy, cystoscopy, gastroscopy, colonoscopy and bronchoscopy) are now augmented by new endoscopic applications because of the promise of shorter recovery times and reduced patient morbidity. Increasing numbers of traditionally open surgical procedures (for example, in intra-abdominal, intra-articular and intra-thoracic operations) are also being done endoscopically.
Aiding this transition are recent dramatic improvements in surgical instruments. An endoscope inserted through a natural orifice or a small incision into a body cavity or potential space gives the surgeon a remarkably clear real-time view of the operative site. The endoscope may incorporate ports for the introduction of other surgical instruments and irrigation fluid, and/or for the removal of waste irrigation fluid, blood and small tissue fragments from the surgical site. One or more additional or alternative ports communicating with the operative site may also be used. The latter ports may comprise natural and/or artificial tracts, passages or orifices, and are typically used in conjunction with an endoscope to facilitate fluid flow to or from the operative site and/or to admit surgical instruments or allow removal of relatively large pieces of excised tissue.
Because of the superior control of fluid flows inherently available in substantially closed surgical cases (including endoscopic procedures), surgeons can theoretically monitor substantially all fluid exchange experienced by a patient during an operation. The potential advantages of such monitoring in reducing the incidence and severity of complications have also led to heightened interest in reexamining fluid monitoring for surgical patients undergoing open procedures. Unfortunately, equipment that would allow such generalized fluid monitoring to be widely practiced has not been commercially available until recently. Among other reasons for its absence are the volumetrically offsetting effects of blood loss and irrigation fluid absorption in a patient, making these latter measurements difficult to determine in light of uncertainty in measured volumes of irrigation fluid dispensed and recovered.
To understand the difficulty, we note that fluid flow control is commonly used to direct and/or maintain a flow of irrigation fluid across the operative site to ensure the surgeon's clear view by splinting the operative space open and/or by continuously carrying away blood (a type of tissue) and small fragments of other types of tissue. By maintaining at least an intermittent flow of relatively low-pressure (frequently electrically non-conducting) irrigation fluid from an external reservoir into the operative site (such as a body cavity), and simultaneously (usually at substantially the same flow rate) withdrawing irrigation fluid from the operative site, optimal operating conditions can be maintained. However, important physiological changes associated with fluid shift may be taking place during the operation; they can seriously affect both inteperative patient safety and the course of postoperative recovery.
For example, while the irrigation fluid flow serves admirably to keep the operative site clear, it tends to mask the true amount of blood loss because lost blood is continuously diluted and carried away. Simultaneously, irrigation fluid and/or components thereof may be absorbed through intact membranes or into portions of the vascular systems (blood and/or lymph) which have been opened by the surgery. The largest component of irrigation fluid (that is, water) is often of greatest concern for reasons described above. But other components, such as local anesthetics which are commonly added to irrigation fluid during liposuction, can be toxic in relatively small quantities when they are preferentially adsorbed or absorbed by tissue surfaces. The total amount of such drugs remaining in the body as well as the time period over which they are accumulated are both important to making informed patientcare decisions related to the surgery.
For example, to optimally maintain a patient's hemodynamic stability during and after an operation, clinically significant volumes of blood lost should be replaced, usually by intravenous administration of crystalloid and/or colloid solutions (possibly including blood and/or blood products) in empirically determined proportions relative to the amount of blood lost. At the same time blood is being lost, however, irrigation fluid is being absorbed. Absorbed irrigation fluid components can expand intravascular volume, reduce the colloid osmotic pressure of the blood, dilute blood coagulation factors, alter the patients' preoperative blood electrolyte concentrations, and in some cases impose a toxic burden (as from excessive local anesthetic). The latter conditions may predispose the patient to further blood loss, pulmonary edema, seizures, coma and death.
Thus, the amount and type of intravenous replacement fluids given (including components such as anesthetics) which can materially alter intravascular volume, fluid distribution, and neurological status, should be governed in part by accurate, real-time assessment of actual blood and other tissue loss, with compensation as necessary for absorption and redistribution of irrigation fluid. In this regard, it should be noted that absorption of irrigation fluid as, for example, through the peritoneum, occurs at a relatively slow and predictable rate, while that through highly vascular tissue undergoing surgery, such as the uterine lining, may be much more rapid. Similarly, significant blood loss may occur over a relatively short period of time (as from a severed artery), or may develop as a prolonged ooze (as from necrotic tissue to be excised). Thus, algorithms for determining the above compensation can be expected to vary with time in any given patient and also to vary with the type of surgery.
For certain patients, such as those in heart or renal failure who may be particularly sensitive to edema and/or changes in intravascular volume, real-time knowledge of the rate of blood loss can reduce the likelihood of intraoperative and/or postoperative complications by allowing adjustment of intravenous fluid administration types and rates to maintain hemodynamic stability. Accurate blood loss estimates can also guide fluid replacement therapy to maintain adequate hemostasis and to improve rheological conditions and oxygenaying capacity for maximizing the flow of well-oxygenated blood to the tissues. It should be noted here that the patient's intraoperative clearance of free water through the kidneys, as well as redistribution of free water within the extracellular space (third spacing fluid in, for example, edema, ascites, and/or pleural effusion) will affect the preferred crystalloid/colloid ratio for intravenously administered fluids. This ratio can be expected to vary over the course of relatively long operations.
Failure to maintain adequate hemostasis can result in catastrophic blood loss, and failure to maintain sufficient oxygenation and/or intravascular volume can result, for example, in shock, possibly leading to tissue necrosis and/or cardiac arrest Excessive intravascular volume, on the other hand, may lead to pulmonary edema and respiratory arrest Whether or not a given patient will actually hemorrhage, arrest or develop any other complication depends on factors such as the patient's initial cardiovascular status, the amount and rate of onset of fluid shifts in the cardiovascular system, and the initial fluid volume status of the patient. Thus, careful monitoring of blood loss and fluid absorption for each patient in real time may help reduce morbidity and mortality when combined with other preoperative and interoperative assessments.
Estimation of Fluid Flow Rates
Conventional measurement of total fluid volume infused through an intravenous catheter is relatively easy, although subject to about 10% reading error when collapsible plastic fluid containers are used. In contrast, estimation of the volume of blood loss is error-prone because a mixture of blood and irrigation fluid may drain substantially continuously from the operative site. Draining fluid is commonly distributed over the surgical drapes, operating table, and floor, as well as to containers resting on the floor. Incidental absorption by and adsorption to various operating-room surfaces, as well as losses in handling due to spillage and splashing, have made irrigation fluid recovery uncertain in the past. Accurate estimation of the blood loss rate and the total blood volume contained in irrigation fluid recovered under these conditions is, of course, problematical, and the clinical usefulness of such estimates is very limited.
A more convenient and more accurate method of estimating blood loss using equipment previously described requires careful collection of waste irrigation fluid. By hanging irrigation fluid source bags and a waste irrigation fluid collector bucket on the same spring scale and then directly and automatically comparing a decrease in source fluid weight with an increase in collected fluid weight, one may estimate net fluid weight gain or loss. Over relatively short periods of time, assuming absorption of irrigation fluid is negligible compared to blood loss, irrigation fluid removed from the source should substantially equal irrigation fluid recovered from the collector. Thus, any indicated weight gain in the source-collector system would be largely due to the volume of blood loss, which is added to the volume of irrigation fluid collected.
This suspended system, while simple, is undesirable because errors in estimated blood loss due to fluid absorption are virtually undetectable. The system is also inconvenient because it is fixed to an overhead support (as, for example, in the ceiling). Additionally, such a suspended system has a tendency to rotate around the suspension member, kinking source and/or drainage tubes and complicating weight measurements because of varying lateral and vertical load components. Accurate weights can only be obtained if the suspended system is actually vertical, but in general it will not be because of the lateral forces applied by the various fluid lines attached to it. Finally, since previously described suspended systems do not provide for independent determination of irrigation source fluid and waste irrigation fluid flow rates and blood loss rate, fluid therapy must be empirical. Without reasonably accurate estimates of absorbed fluid and blood loss flow rates, the optimal balance of crystalloid, colloid and blood products in the fluid therapy for each surgical patient becomes a clinical judgment subject to considerable error. Further, the presence of tissue other than blood (such as fat) in waste irrigation fluid can compound errors in estimates of blood loss and fluid absorption.